Field of the Invention
The invention relates to a flowmeter for determining the flow of a multiphase medium flowing through a measuring tube having a measuring device implementing a tomographic measuring principle. The invention also relates to a method for operating such a flowmeter.
Description of Related Art
The atomic nuclei of the elements that have nuclear spin also have a magnetic moment caused by the nuclear spin. The nuclear spin can be regarded as angular momentum describable by a vector and correspondingly, the magnetic moment can also be described by a vector, which is oriented parallel to the vector of the angular momentum. If a macroscopic magnetic field is present, the vector of the magnetic moment of the atomic nucleus tends to orient itself parallel to the vector of the macroscopic magnetic field at the atomic nucleus. Here, the vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the atomic nucleus. The frequency of the precession is called Larmor frequency ωL and is proportional to the magnitude of the magnetic field strength B. The Larmor frequency is calculated according to ωL=γ·B where γ is the gyromagnetic ratio, which is at a maximum for hydrogen atoms. The gyromagnetic ratio indicates the proportionality factor between the angular momentum or the spin of a particle and the associated magnetic moment.
Measurement and analysis methods that use the properties of precession of atomic nuclei having a magnetic moment when a macroscopic magnetic field is present are called nuclear magnetic resonance measurement or analysis methods. Nuclear magnetic resonance is abbreviated to NMR.
An important representative of the measuring principles is magnetic resonance tomography, also called magnetic resonance imaging, MRI. Normally, electric signals induced by the precessing atomic nuclei under different limiting conditions in a sensor coil are used as output variable for the measurement and analysis method.
An example of measuring devices that use magnetic resonance are nuclear magnetic flowmeters, which measure the flow of a multiphase medium flowing through a measuring tube and analyze the medium.
A requirement for analysis using nuclear magnetic resonance is that the phases of the medium to be analyzed are able to be excited into distinguishable nuclear magnetic resonances. The analysis can include the flow velocity of the individual phases of the medium and the relative fractions of the individual phases in the multiphase medium. Nuclear magnetic flowmeters can, for example, be used for analysis of multiphase mediums extracted from oil sources. The medium then consists essentially of the phases crude oil, natural gas and salt water, wherein all phases contain hydrogen atomic nuclei.
The analysis of the medium extracted from oil sources can also take place using so-called test separators. These channel off a small portion of the extracted medium, separate the individual phases of the medium from one another and determine the fractions of the individual phases in the medium. However, test separators are not able to reliably measure crude oil fractions of less than 5%. Since the crude oil fractions of many sources is already less than 5%, it is not possible at this time to economically exploit these sources using test separators. In order to further economically exploit sources with a very small crude oil fraction, accordingly exact flowmeters are necessary.
Normally, electric signals induced by the precessing atomic nuclei after excitation in a sensor coil are used as output variable for evaluation. A requirement for the measurement of a multi-phase medium is, as already mentioned, that the individual phases of the medium can be excited to distinguishable nuclear magnetic resonances. The magnitude of the electric signals induced by the precessing atomic nuclei of one phase of the medium in the sensor coil is dependent on the number of precessing atomic nuclei per volume element in this phase, thus depending on the density of the phase, but also on the influence time of the precessing atomic nuclei in the influencing, controlled magnetic field. Thus, the magnitude of the induced electric signal in the liquid phases is greater than in the gaseous phases.
Spatial information necessary for magnetic resonance imaging is, for example, applied to the sample with a gradient field. Since the Larmor frequency of the atomic spin is proportional to the magnetic field strength, a location-dependent distribution of different Larmor frequencies of the atomic spins is created by the gradient field and thus a spatial dependency of the electric signals induced by the atomic nuclei.
As described above, the MRI signal is dependent on the density of the medium. In a comparison of the average values of the signal amplitudes per cubic meter of gas, oil and water, it can be determined that the signal from gas is clearly different than that of oil and water, however, there is almost no difference between the signals from oil and water. The strength of the signal can be expressed by the so-called hydrogen index HI. The hydrogen index HI describes the relative fraction of hydrogen atoms of a medium compared to water. Accordingly, the hydrogen index of water HIwater=1. The indices for oil and gas are HIoil=0.9-1.1 and HIgas=0-0.2. With the help of the MR signals, it is easy to distinguish gas, on the one hand, and liquid (consisting of water and oil), on the other hand. Differentiating between water and oil is difficult or very complex, since the amplitudes of the MR signals are barely different.
As already described, nuclear magnetic measurement and analysis methods are based on the effect that the magnetic moments of the nucleus are aligned along the field line of an externally applied magnetic field. This leads to a bulk magnetization of the medium. The rate at which this magnetization establishes is determined by the so-called spin lattice relaxation time T1 and has an exponential course.
A further measurement variable typical for nuclear magnetic measurement and analysis methods is the spin-spin relaxation time T2. This time is a measure for inhomogeneity in the magnetic field surrounding the one single spin.
The mechanisms, which determine the values for T1 and T2, are dependent on the molecular dynamics of the test sample. The molecular dynamics are, in turn, dependent on the size of the molecules and also on the intermolecular spacing. These are different for each medium. Accordingly, different mediums also have different values for T1 and T2.
A measurement method known from the prior art for characterizing individual phases of a multi-phase medium is given by the measuring principle of pre-magnetization contrast measurement. This measuring principle is based on the difference in the T1 time for different phases of a multiphase medium and is suitable in a distinct manner for determining the oil fraction and the water fraction as well as the relative ratio of the oil fraction to the water fraction in a sample.
The multiphase medium flows through a section interfused with a constant magnetic field. Here, the magnetic field has at least one component perpendicular to the direction of flow of the medium. Since the alignment of the magnetic moments in the magnetic field is dependent on the respective phase of the medium, different formation of magnetization in the individual phases results at the same exposure time. The exposure time of the magnetic field is determined by the length of the section interfused by the constant magnetic field and the flow velocity of the medium.
In general, the longitudinal relaxation time T1 of oil is much smaller than that of water. Accordingly, the magnetization of oil parallel to the outer magnetic field establishes more quickly than for water. By varying the length of the pre-magnetization section, the signals from oil and water are each formed at a different level, so that the ratio of oil fraction to water fraction in the medium can be determined from the oil-water signal ratio dependent on the pre-magnetization section. The strong contrast between oil signal and water signal depending on the pre-magnetization section offers a good possibility for determining the oil to water ratio (OWR) of the medium.
Since the signal of the gas fraction is very weak, the method is, on the one hand, independent of the gas fraction. However, on the other hand, it is not suitable for determining the gas fraction, so that not all three phases of the medium can be characterized using the measuring principle of pre-magnetization contrast measurement.
Another measuring principle, which is also often used in flow measurement technology and is not based on nuclear spin resonance is by electrical capacitance tomography (ECT).
Electrical capacitance tomography is a method known from the prior art for measuring and characterizing multiphase media. It is generally suitable for dielectric materials and is based on the fact that different materials have different permittivities.
A typical measuring device for electrical capacitance tomography is designed in such a manner that a certain number of electrodes are arranged around a measuring tube. Measuring devices known from the prior art usually have eight, twelve or sixteen electrodes.
In a measuring device of the type being described, an excitation voltage is applied to an electrode and the induced voltage/the current is measured in all other electrodes, while their electric potential is kept at zero. This is carried out for all existing electrodes. Using the example of a measuring device with eight electrodes, the first electrode is used in a first step as excitation electrode and the second to eighth electrodes are used as detector electrodes. In the next step, the second electrode is used as an excitation electrode and the third through eighth electrodes are used as detector electrodes, etc. In a measuring device with N electrodes, there are N·(N−1)/2 electrode pair combinations, and thus, N·(N−1)/2 measuring values of capacity from which an image can be constructed. The construction occurs by means of an evaluation algorithm, which is not explained in detail here.
Since the capacity is dependent on the permittivity, i.e., the permeability of a material for an electric field, of the multiphase medium between the electrodes, it is thus possible to dissolve the distribution of the individual phases using the measured values, since each phase of the medium has a different permittivity.
The permittivity of gas is about 1, εr≈1, the permittivity of oil between 2 and 4, εr≈2-4, and the permittivity of water is greater than 50, εr>50. Using the values shown here for the permittivity of the individual phases, it can be observed that it is very difficult and complex to separate the gaseous phase from the oil phase, since the values of permittivity characterizing the two phases are not far from one another, namely almost the same. Electrical capacitance tomography was shown above to be a good method for determining the hydrocarbon fraction of a multi-phase medium, which is made up of the oil fraction and the gas fraction, and the water fraction of the medium.
The measuring principles described above, as shown, have great advantages in the measurement of certain properties of a multiphase medium. On the other hand, however, they also have the shown disadvantages or limitations so that the determination of all three phases of the multiphase medium is either not possible, inexact or extremely complex.